The Liquid Chalk by DMM. Liquid chalk gives a much better coverage than standard loose chalk and takes longer to wear off, i.e. you will be able to do more moves before your fingers start to turn greasy. This can be a great help on long boulder problems and traverses or on sport routes where strength sapping pauses to chalk up need to be kept to a minimum.
Ideally it is used in conjunction with standard chalk. Think of it as an extra boost for those big redpoint attempts. That said, the alcohol solution does help to clean skin, thus making it a useful application at the start of a session when your hands may be carrying greasy deposits from whatever you have been doing prior to climbing.